To cut or drill a brittle workpiece such as stone, bricks, concrete, and asphalt, an abrasive material having a higher hardness than the workpiece is required.
As the abrasive material, there are synthetic diamond particles, natural diamond particles, boron nitride particles, and cemented tungsten carbide particles. The synthetic diamond particles are most generally used.
The synthetic diamond (hereinafter, referred to as a “diamond” is invented in 1950s, known as a material whose hardness is highest from materials existing on the earth, and used for cutting or grinding tools due to the characteristics.
Particularly, the diamond is generally used in a stone processing field of cutting or grinding granite and marble and a construction field of cutting or grinding concrete structures.
The cutting or grinding tools include a cutting tip including diamond particles for cutting a workpiece and a bonding material for maintaining the diamond particles.
Most of cutting tools have been formed by a powder metallurgy method of mixing, compacting, and sintering diamond particles with metal powder as a bonding material.
A cobalt powder has been most generally used as a bonding material for diamond cutting tools for a long time.
The cobalt bonding material is called as “an all-around bonding material” in the field of diamond tools because a cutting tip formed using a cobalt bonding material has an excellent cutting speed regardless of a workpiece such as granite, concrete, asphalt, and marble or whether horsepower (HP) of a cutting machine is high or low.
Since cobalt bonding materials are well abraded, diamond particles are well projected, thereby making cobalt bonding materials in the limelight as all-around bonding materials.
Since abrasion of a bonding material well occurs in small low power cutting machine of 2 to 3 HP, bad cutting does not occur.
However, since cobalt bonding materials are so quickly abraded in cutting machine of high HP, cutting speed is high but diamond particles are too early departed, thereby shortening a lifetime.
Recently, high HP machine is introduced and used for improving productivity of cutting granite, concrete, or asphalt.
In granite cutting factories, though large-sized multi-blade machine of 100 HP has been used during past 10 years, machine of 150 HP is generally used and machine of 200 HP is introduced now.
On the other hand, to improve cutting productivity, machine of 20 HP has been replaced with machine of 40 or 65 HP for cutting concrete or asphalt pavements. Even machine of 100 HP is used.
As described above, as a HP of cutting machine increases, a lifetime of cutting tool cannot be guaranteed by using a bonding material of pure cobalt.
A method of adding tungsten (W) and tungsten carbide (WC) is generally employed for slowing down abrasion of the cobalt bonding material.
Recently, to extend a lifetime of the bonding material, an amount of adding tungsten carbide has to be increased to 50 to 60%.
However, as the amount of adding tungsten carbide increases, problems as follows occur.
When cobalt (Co) and tungsten carbide (WC) form a bonding material, a sintering temperature has to be raised over 1000° C. to sinter the bonding material when an amount of tungsten is more than 50%.
As the sintering temperature is raised, thermal deterioration of diamond particles mixed with the bonding material cannot be avoided.
As the sintering temperature is raised, diamond particles are transformed to graphite and hardness of the diamond particles is rapidly decreased.
Therefore, in a diamond tool industry, it is tried to lower the sintering temperature to be less than 900° C. and a sintering temperature more than 1000° C. is avoided as far as possible.
This is because excellent cutting speed and a long lifetime cannot be acquired when the thermal deterioration of diamond particles gets worse.
Accordingly, the amount of tungsten carbide is reduced not to raise the sintering temperature. However, when the amount of tungsten carbide is reduced, the abrasion of cobalt cannot be slowed down.
Also, a cobalt bonding material is expensive, variation of the price of the cobalt bonding material is great, and there exists an environmental problem.
Accordingly, a lot of efforts for replacing the cobalt bonding material with another have been done. On the other hand, since iron is cheap and there are relatively few environmental problems, iron draws attentions to substitute for cobalt.
However, in the case of iron on the market, though carbonyl iron powder having a micro particle size is used, it is hard to get densified microstructure after sintering. Therefore, a high temperature of 1000° C. is required to raise a sintering density.
However, when a sintering temperature is raised, diamond particles mixed with a bonding material are transformed into graphite, thermal deterioration of diamond, in which strength of the diamond is rapidly dropped, is accelerated. When thermal deterioration of diamond particles gets worse, it is hard to get excellent cutting speed and a long lifetime.
Therefore, in a diamond tool industry, it is tried to lower the sintering temperature to be less than 900° C.
Also, due to physical properties such as hardness and a transverse rupture strength lower than the cobalt, a bonding material formed by sintering, iron powder is inferior in mechanical retention force for diamond and abrasion is not smooth to lower cutting, speed, thereby restricting application to diamond tools.